The present invention relates to a method for controlling an internal combustion engine and more particularly to a method for adjusting spark based on the number of fueled cylinders during a start.
Engine starting control has a significant impact on engine emissions and engine speed run-up. Spark placement, relative to piston position, influences both torque and emissions. Torque is necessary to accelerate an engine from cranking speed up to idle speed. Further, low starting emissions are desirable when catalysts are cold and their efficiency is low. In general, advancing spark increases engine torque while retarding spark reduces emissions. Therefore, it is important to provide consistent well-placed spark timing to ensure engine speed run-up with reduced emissions.
One method to adjust spark while an engine is cold is described in U.S. Pat. No. 6,135,087. This method provides spark advance based on coolant temperature and engine speed. Further, the amount of spark advance accounts for engine position and time from the start-to-run transfer. More particularly, the method initially determines whether the desired spark advance is before top dead center and whether the throttle is open. If so, the method uses engine speed and coolant temperature to determine a spark advance multiplier. Thereafter, the current engine position pulse is loaded and an engine position multiplier is interpolated and applied to the spark advance multiplier value. Next, the time since the start-to-run is loaded and a start-to-run multiplier is interpolated and applied to the spark advance multiplier value. Finally, the spark is advanced via the spark advance multiplier value as adjusted by the engine position pulse multiplier and the time since start-to-run transfer multiplier. Upon engine operation reaching an after top dead center condition or when the throttle is closed, the method is exited and the engine is returned to normal spark control.
The inventors herein have recognized several disadvantages of this approach. Namely, the approach changes spark advance based on engine position, thereby resulting in inconsistent spark placement during the engine start. In other words, when an engine is turned off, it generally stops at a random position. In general, key-off removes power from the engine controller and sensors so that engine position data is lost. Consequently, the engine controller monitors several signals during a start to reestablish engine position. Thus, engine position is changing while the engine controller monitors cam and crank signals, attempting to determine engine position. The number of cylinder events before engine position can be established will vary from start to start depending on where the engine has stopped and on the complexity of the engine position monitoring system.
Therefore, if spark based on position is delivered without regard to synchronization between the engine controller and the engine, or without regard to fuel delivery, the angle at which spark is delivered may vary from start to start.
As an example, a fueled cylinder receiving spark may receive spark at an angle intended for the next or prior fueled cylinder. As such, engine position based spark as presented in the prior art, may deliver less than optimal spark.
Furthermore, the method functions only when base spark advance is after top dead center (ATDC) and if the throttle is open. Therefore, the above-mentioned approach does not optimally deliver spark during start where the throttle is closed and retarded spark is used to lower emissions.
Another method to adjust spark when and engine is cold is described in U.S. Pat. No. 5,483,946 owned by the assignee of the present invention. The method describes retarding ignition timing from a nominal value during a period following engine start and returning the ignition timing to the nominal value by termination of the period, where the period is based on time.
The inventors herein have also recognized that while this approach works well during cold engine operation, it can be inaccurate during start because the method adjusts spark in relation to time. Spark based on time delivers a spark advance that is a function of time since the timer is started. However, there is not a one to one relationship between engine position and time due to variability in engine stopping location as discussed above. Further, engine position is a mechanical dimension; time is a continuum, which lacks spatial dimensions.
One embodiment of the present invention includes a method that improves spark placement and consistency during start is presented. The method comprises: counting a number of cylinders receiving at least one fuel injection from a start of an internal combustion engine; and calculating cylinder spark advance based on said counted number of cylinders. This method can be used to reduce the above-mentioned limitations of the prior art approaches.
By counting the number of cylinders that have received fuel, and delivering spark based on the cylinder count, the inventors herein have improved engine starting. Independent of engine stopping location, the first fueled cylinder and subsequent cylinders will receive consistent-spark, start after start. For this reason, the engine controller can deliver fuel immediately or delayed and still maintain combustion consistency during a start. This can be used to produce low emissions and uniform engine speed run-up.
Further, another advantage of the present invention, derived from counting fueled cylinders, is that a better match between cylinder mixture and spark advance is possible. The inventors herein have recognized that during a start, changes occur within an engine and its surroundings. The first few fired cylinders have an air-fuel mixture that is composed of fresh charge and fuel. In other words, there is very little EGR or residuals during the first few combustion events. After the first few cylinders fire and expel their residuals, the residuals affect air-fuel mixtures in other cylinders. Therefore, the combustion process in an engine is not linked to time, but to the number of cylinders receiving fuel.
Furthermore, since the cylinders receiving fuel have distinct air-fuel-residual mixtures, it is desirable to provide spark suited to these mixtures. Spark delivery based on the number of fueled cylinder events allows the engine controller to deliver unique spark angles to individual cylinders. This allows the engine controller to account for individual cylinder air-fuel mixture differences.
In addition, fuel composition also affects mixture preparation and may influence engine speed run-up. Fuels containing alcohol provides less energy affecting torque and engine speed. If spark is delivered based on engine speed and load, the controller may alter the spark in an undesirable manner. Therefore, spark delivery that solely or additionally takes into account the counted number of fueled cylinders can be used to improve spark placement consistency with regard to engine control and combustion mixtures.
The present invention provides a number of advantages. The present invention provides the advantage of improved spark control during engine starting, resulting in lower emissions. This advantage is especially beneficial when a catalyst is cold and its efficiency is low. In addition, the present invention improves engine speed run-up consistency. Repeatable engine speed during starting improves owner confidence and satisfaction since the engine behaves in a reliable and predictable manor.